1. Field of the Invention
This invention relates generally to apparatus and methods for designing MEMS based optical and photonic devices for fiber optical transmission systems. More particularly, this invention is related to the optimization of the configuration of optical and photonic variable attenuators for use in optical communication sub-system or system.
2. Description of the Related Art
Even that early development of mechanical variable attenuator, as that disclosed in U.S. Pat. No. 6,144,794, entitled xe2x80x9cMotor driven optical fiber variable attenuatorxe2x80x9d, provides good optical performances and compact mechanical size, which is better than many other mechanical variable attenuators in the market, this type of attenuators have several limitations. Specifically, the patented mechanical variable attenuator comprises a motor, which actuates a lead screw to move rotatably, and a nut associated with the lead screw to move linearly. A neutral density filter moves along with the linearly moving nut wherein the filter is positioned between two spaced collimators for providing linear attenuation changes of light transmitted therebetween. Due to the mechanical structure and designs, the device has intrinsic disadvantages of poor reliability, high power consumption, low tuning speed and large size. A group new technical approaches based on micro electromechanical system (MEMS) are launched to achieve the production of optical devices with low power consumption, small size, high speed and high reliability.
Recent developments of the mechanical anti-reflection switch (MARS) for implementation as optical variable attenuator are still faced with a technical limitation. The configuration is mostly employed as a single layer membrane and silicon substrate, which forms a Fabry-Perot resonator. Static electronic force causes the membrane move back and forth, so that the gap of the resonator formed by the membrane and the substrate changes. The attenuation of the light within one certain wavelength range is achieved due to close to or far away from the resonance frequency. However, the MARS design is suffered of large insertion loss, extremely non-linear voltage/attenuation curve and relatively high tuning voltage. These are all design intrinsic problems according to current state of art configurations. Basically, the single layer of silica nitride with low refraction index does not have high reflection rate, e.g., 30%. Accordingly, a high tuning voltage must be applied. Secondly, though the design is to achieve the same reflection rate, the uncontrollable manufacture process-deviations cause reflection rate differences, which dramatically affect the insertion loss performance of the device, especially when the reflection rate is designed to be higher. Thirdly, MARS designs are implemented with a configuration to place the output port along the reflection direction. Theoretical analyses prove that such configuration has poor linearity of voltage/attenuation curve and very large residual insertion loss.
Based on the principle of the well-known Fabry-Perot interferometer constructed by Charles Fabry and Alfred Perot in 1897, Goossen discloses in U.S. Pat. Nos. 5,943,155 and 5,949,571, entitled xe2x80x9cMARS Optical Modulatorsxe2x80x9d, a double polysilicon MARS (mechanical anti-reflection switch) device where the shorting between the lower polysilicon metalization and the silicon substrate is prevented by providing an insulating layer on the surface of the silicon substrate. Goossen teaches a modulator configuration using the reflection modulation of the Fabry-Perot interferometer with improved reliability. The disclosures made by these prior patents are hereby incorporated as reference in this Application. The configuration disclosed by Goossen however is far away from an optimized design structure according to above descriptions of the technical difficulties now still encountered by those of skill in the art in MARS design and manufacture.
Another recent development for variable attenuator device is by fabricating a group of membrane-based strips. David M. Bloom disclosed in U.S. Pat. No. 6,215,579: Method and apparatus for modulating an incident light beam for forming a two-dimensional image. The apparatus includes a plurality of elongated elements each having a reflective surface. The elongated elements are suspended substantially parallel to each other above a substrate with their respective ends supported, forming a column of adjacent reflecting surfaces grouped according to display elements. Alternate ones of each group are deformable by a applying a voltage with respect to the substrate. An approximately flat center section of each deformed element is substantially parallel to and a predetermined distance from a center section of each undeformed element. A light beam incident to the column of adjacent reflecting surfaces is reflected from a group of elongated elements when the alternate ones are undeformed and diffracted when alternate ones are deformed. A distance of movement is controlled or a ratio of between reflection and diffraction periods determines the display intensity for the corresponding display element. Diffracted light is collected by a lens and reflected by a scanning mirror into an eyepiece or onto a display screen.
The same apparatus of the invention is used to make a variable attenuator device. These strips form an optical grating. Each individual strip can be tuned by electronic static force. The strips are divided into two groups. The input light hits the strips at almost a normal direction and responses back to output. Since one group of the strips can be changed due to electronic static force, the interference of the two groups will cause the power variation according to the wavelength change of input light or the distance change of the two strip groups. A variable attenuator device can be achieved by means of this method. One key disadvantage of this structure is the grating is not high efficient device. Between the two adjacent strips, there should be a gap, so that the strip can move freely without touch each other. The gap will cause attenuation to the light that passes through thus increases the basic insertion loss of attenuation device. Furthermore, the device has another disadvantage that there is a non-linear relationship between voltage and attenuation. Based on the interference theory, the linearity variations can cause extreme adversity to optical performance at the high attenuation area, which in turn causes control problem for electronic circuit.
The third MEMS approach for variable attenuator device is to implement micro-mirrors in optical devices. According to the data from research and prototyping sources, the poor reliability of these micro mirrors in these types of devices become a critical concerns preventing the micro-mirror based MEMS optical devices from practically useful. There are mainly two reasons that cause the poor reliability performance. First, the mirror is supported on a hinge. Manufacture of a mirror hinge is often becomes the most difficult step in applying a wafer-based process for producing a MEMS device. Furthermore, as the micro mirrors rotate around the hinge, electrical charges and heat are generated. It becomes very serious when the mirror rotates for million times. Mirror also needs a relative large moving range, so that the light can be attenuated to a value required by an optical signal transmission system. The large tuning range worsens the difficulty by further degrading the already-poor reliability. According to results of the simulation analyses, most MEMS mirror structure is with a natural frequency of several KHz. This means it is very difficult to pass the vibration test that is necessary for the application of components in telecommunication system. In general, the mirror-based MEMS device is faced with tremendous problems of poor reliability, instability due to sensitivity to vibrations and slow tuning speed.
Therefore, a need still exists in the art in the field of the optical signal transmission systems to provide a configuration and method of manufacture to provide an optimized variable attenuator such that the limitations encountered by current MARS configuration can be overcome. It is further desirable that device is controlled by smart control system to compensate the attenuation while temperature and wavelength shift, so that the attenuation can be fixed regardless the outside environment or input wavelength.
It is therefore an object of the present invention to provide new and improved configuration and methods for optimized design and manufacturing of MEMS based optical/photonic attenuators controlled by separate control circuits. The innovative design includes: multiple layers are made to form the membrane with high reflection rate which forms the resonator; two identical membrane are designed by the same fabrication process; transmission port is used as output port of device; the residual basic mode of MEMS resonator is designed to match the input mode; anti-reflection coating on every interface in the optical path; the conductive is chosen as the layers face to face, there should be no dielectric material to trap the charges between the positive and negative electrodes; an electronic control circuit with temperature and wavelength compensation functionality makes the MEMS device with fixed attenuation at one certain desired attenuation value without variation according to the temperature and wavelength change.
These new configurations produce an optical variable attenuator device with low insertion loss, low wavelength dependent loss (WDL), low temperature dependent loss (TDL), low ripple, and large attenuation range with low voltage range. The present invention also tolerant relatively large manufacturing process-deviations and hence significantly reduces the production cost.